The overall objective of this research is to understand the mechanism for and the regulation of the enzymatic synthesis of cholesterol, triglycerides and other water-insoluble substances by the liver. Specifically, we have identified and purified two substrate specific sterol carrier proteins (SCP1 and SCP2), isolated from the 105,000 x g supernatant (S105) of rat liver, which are required for liver microsomal cholesterol synthesis. SCP1 is required for the microsomal conversion of squalene to lanosterol. SCP2 is required for the microsomal conversion of 4,4-dimethyl-delta 8-cholestenol to C27-sterols and for the microsomal conversion of 7-dehydrocholesterol to cholesterol. We have devised a method which is capable of endogenously generating microsome-bound squalene from the water-solble precursor, farnesyl pyrophosphate. Microsome-bound squalene also requires SCP1 for its conversion to lanosterol. The requirement for a specific supernatant protein, e.g., SCP1, is first seen only after the appearance of squalene, the first water-insoluble intermediate in cholesterol biosynthesis. Using microsome-bound squalene as well as other microsome-bound sterol intermediates, studies will be conducted concerning the mechanism of interaction between SCP1 and SCP2 and their respective membrane-bound intermediates. Present evidence is compatible with the proposal that SCP1 facilitates the movement of squalene either on or within the microsomal membrane. Studies will also be coonducted to determine a possible regulatory role of SCP1 and SCP2 in cholesterol biosynthesis. We have also demonstrated a requirement for a soluble protein or proteins for the microsomal conversion of palmityl-CoA to triglyceride and for the microsomal conversion of diolein to triglyceride. Studies will be conducted to purify the protein or proteins and to determine their mechanism of action. Since the stability of cholesterol in the blood is of crucial importance in the disease atherosclerosis, the ability of sterol carrier proteins to maintain sterols in a soluble form during membrane-bound biosynthetic reactions should be of both scientific and medical significance.